Php 如何将同一mySQL表中的多个数据左键联接？

这是我的桌子位置：

这是我的餐桌上的人：

我从mySQL数据库发出的请求

$pdo = $db->query('
SELECT *, position.id AS id, people.id AS people_id 
FROM position 
LEFT JOIN people 
ON position.teacher=people.id;
ON position.cook=people.id;
ON position.doctor=people.id;
ON position.dentist=people.id;
');

while ($row = $pdo->fetch(PDO::FETCH_ASSOC)) {
echo "The teacher is "$row['firstname']." ".$row['name'];
echo "The cook is "$row['firstname']." ".$row['name'];
echo "The doctor is "$row['firstname']." ".$row['name'];
echo "The dentist is "$row['firstname']." ".$row['name'];
}

我的结果是：

The teacher is Ben Harper
The cook is Ben Harper
The doctor is Ben Harper
The dentist is Ben Harper

我需要的结果是：

The teacher is Ben Harper
The cook is Samanta Jones
The doctor is Emiliy Rose
The dentist is Fred Miller

试试这个

$pdo = $db->query('
SELECT *, position.id AS id, people.id AS people_id,
(case when position.teacher <> '' then 'teacher'
when position.cook <> '' then 'cook'
when position.doctor <> '' then 'doctor' 
when position.dentist <> '' then 'dentist') position_name
FROM position 
LEFT JOIN people 
ON position.teacher=people.id;
ON position.cook=people.id;
ON position.doctor=people.id;
ON position.dentist=people.id;
');

while ($row = $pdo->fetch(PDO::FETCH_ASSOC)) {
echo "The ".$row['position_name']." is "$row['firstname']." ".$row['name'];
}

试试这个

$pdo = $db->query('
SELECT *, position.id AS id, people.id AS people_id,
(case when position.teacher <> '' then 'teacher'
when position.cook <> '' then 'cook'
when position.doctor <> '' then 'doctor' 
when position.dentist <> '' then 'dentist') position_name
FROM position 
LEFT JOIN people 
ON position.teacher=people.id;
ON position.cook=people.id;
ON position.doctor=people.id;
ON position.dentist=people.id;
');

while ($row = $pdo->fetch(PDO::FETCH_ASSOC)) {
echo "The ".$row['position_name']." is "$row['firstname']." ".$row['name'];
}

---

如果你重组你的桌子会更好

+----+------------+ | id | position | +----+------------| | 1 | teacher | | 2 | cook | | 3 | doctor | | 4 | dentist | +----+------------+ +----+-----------+--------+-----+-------------+ | id | firstname | name | age | position_id | +----+-----------+--------+-----+-------------+ | 1 | Fred | Miller | 42 | 4 | | 2 | Emily | Rose | 32 | 3 | | 3 | Ben | Harper | 38 | 1 | | 4 | Samanta | Jones | 35 | 2 | +----+-----------+--------+-----+-------------+

---

如果你重组你的桌子会更好

+----+------------+ | id | position | +----+------------| | 1 | teacher | | 2 | cook | | 3 | doctor | | 4 | dentist | +----+------------+ +----+-----------+--------+-----+-------------+ | id | firstname | name | age | position_id | +----+-----------+--------+-----+-------------+ | 1 | Fred | Miller | 42 | 4 | | 2 | Emily | Rose | 32 | 3 | | 3 | Ben | Harper | 38 | 1 | | 4 | Samanta | Jones | 35 | 2 | +----+-----------+--------+-----+-------------+

---

正如上面的注释和答案中所指出的，如果您可以重新构造表，那么您绝对应该—正如其他人所提到的，最好将表重新构造为具有额外的列，这样您的查询将更加简单和高效。但是，如果您陷入了一个无法解决的境地，并且您只有少量的数据，因此这一问题的低效性不会引起太大的关注，那么您可以使用一个联合来将几个简单的查询粘合在一起：

Select position.id, firstname, lastname, 'cook' AS position
FROM position LEFT JOIN people ON position.cook = people.id

UNION

Select position.id, firstname, lastname, 'teacher' AS position
FROM position LEFT JOIN people ON position.teacher = people.id

UNION
...

---

同样，这不是一种有效的方法-如果可以的话，应该重新构造数据。但是，如果出于任何原因，您不能这样做，那么这将使您以一组行的形式返回所需的数据，从而返回您所需的数据。

正如上面的注释和答案中所述，如果您可以重新构造表，您绝对应该—正如其他人所提到的，最好重新构造表以获得额外的列，这样，您的查询将更简单、更高效。但是，如果您陷入了一个无法解决的境地，并且您只有少量的数据，因此这一问题的低效性不会引起太大的关注，那么您可以使用一个联合来将几个简单的查询粘合在一起：

Select position.id, firstname, lastname, 'cook' AS position
FROM position LEFT JOIN people ON position.cook = people.id

UNION

Select position.id, firstname, lastname, 'teacher' AS position
FROM position LEFT JOIN people ON position.teacher = people.id

UNION
...

---

同样，这不是一种有效的方法-如果可以的话，应该重新构造数据。但是，如果出于任何原因，您不能这样做，那么这将使您以一组行的形式返回所需的数据，从而返回所需的数据。

更改表结构如何：1-将位置表更改为id，位置名称2-向人员表位置id添加额外列。。那么你有更简单的问题，可能是个主意@Jarla这是唯一的想法。@草莓rahul_m给出了第二个想法。哎呀，这是一个糟糕的想法。更改表结构如何：1-将位置表更改为id，位置名称2-向人员表位置_id添加额外的列。。那么你有更简单的问题，可能是个主意@“贾拉，这是唯一的主意。”草莓拉胡尔·m说了第二个主意。哎呀，这是个糟糕的主意。